While animals could never live by themselves, some autotrophic bacteria can.

A community of several different kinds of bacteria would have better chances than a single species, but for bacteria there is certainly no need for thousands of species.

Autotrophic bacteria would need only an environment providing less than 20 essential chemical elements (most of which belong to the most abundant elements, a notable exception being molybdenum) and either solar light for energy, neither too little nor too much, or a chemical source of energy, like dihydrogen + carbon dioxide, which can be provided by volcanic gases or by the reaction of water with volcanic rocks.

There would have been many places in the Solar System suitable for bacteria, except that where there is water, it is usually too cold, and where it is not too cold, there is no water.